In the recent years, there has been a constant increase in the tendency of petroleum refiners to implement delayed coking process as part of their overall operation of processing of the crudes, because of the advantages it is known to provide. Further, with the crude sources becoming heavier or with the more refiners switching to processing “opportunity crudes” (also referred in the industry as challenging crudes), it is anticipated that more interest will be shown in delayed coking processes. In Delayed Coker Unit (DCU), a heavy hydrocarbon feedstock is fed to a furnace, which heats the feedstock to the desired coking temperature and is designed and controlled to prevent premature coking in the heater tubes. The hot feedstock is then passed from the heater to one or more coker drums where the hot material is held for an extended period of time at desired pressure, until coking reaction completes. Vapors from the drums are fed to a fractionator where gas, naphtha, and gas oils are separated out. The heavier hydrocarbons obtained in the fractionator are recycled through the furnace as per the requirement. After the coke reaches a predetermined level in one drum, the feed flow is diverted to another coker drum to maintain continuous operation. The coked drum (i.e. coker drum having coke upto the predetermined level) is steamed to strip out entrapped hydrocarbons, cooled by water injection and decoked by mechanical or hydraulic methods.
Recently in prior art, a number of inventions have come up in the area of delayed coking process, that suggest addition of some external additive(s)/chemicals to the coker feedstock in order to meet various objectives like reduction of coke yield, improving the quantity as well as quality of liquid and gaseous products and improving the quality of coke produced. By way of example, U.S. Pat. No. 4,378,288 describes a method for increasing the distillate yield in delayed coking process by adding a free radical inhibitor to the coker feed material. U.S. Pat. No. 4,642,175 describes a process for upgrading the heavy hydrocarbon feedstock by reducing the coking tendency by contacting with free radical removing catalyst. U.S. Pat. No. 4,756,819 tries to prevent the coke formation in thermal treatment of heavy hydrocarbon residues by use of a metallic salt in the form of suspension of solid particles, in solution or as emulsion. U.S. Pat. No. 5,006,223 describes a method of increasing the thermal conversion of hydrocarbons without any substantial increase in gaseous products formed, by the addition of certain free radical initiators.
In the aforesaid documents, the additives are added to the feedstock at a stage before the feedstock is fed to the coker drum. Residence time of the additive in the process is increased by incorporation of the additives in the feedstock before the same is fed to the coker drum. This may lead to reduction in activity of the additive. Moreover, the presence of additives in the furnace tubes may lead to increase in the possibility of coke deposition on the metal surface.
Reference may be made to U.S. Patent Publication No. 2009/0209799 that describes a process in which the hydrocarbons are cracked or coked by adding an additive into the vapors emerging from the coker drum or coking vessel. Particularly, the document describes methods for injecting the additives into the vapor phase at an upper portion of the coker drum. It is felt that since the coking reactions predominantly take place in the liquid pool such a procedure may not be providing the best results. Thus, there exits a need to improvise the delayed coking process used in petroleum refineries for one or more of the following objectives (a) reduction of coke yield, (b) improving the quality and quantity of liquid and gaseous products or (c) improving the quality of coke produced, including coke morphology.